Electric induction apparatus



Nov. 7, 1950 G. czAMlLLlv 2,529,135

' 1 ELECTRIC INDUCTION APPARATUS Filed Dec. 22. 1948 za Figi. '50 2Q PRESSURE Inventor: Guglielmo Camll,

TIME- y Attorney.

PRESSURE -b Patented Nov. 7, 150

2,529,135 .ELECTRIC INDUCTION APPARATUS l Guglielmo Camilli, Pittsfield, Mass., assigner to General Electric New YorkV Company, a corporation of Application Decei'n'tr'zz, 194s, serial No. 66,806

E`This1invention relates to electric induction apparatus and more particularly to improvements inporcelain-clad-liquid lled instrument trans` formers. H

`By the term porcelain I wish to include all brittle'cerainic and vitreous dielectric materials. In order to reduce the overall height of high voltage instrument transformersat least a part of the transformer proper, vthat is to say, the core and coil assembly, often is mounted Within a.cylindrical' porcelain insulator which constitutes Vthe main dielectric material between the high voltage line terminals of the'transformer andV the'portions of the transformer which are normally Vat fgroundfpotential. It is customary in many instances substantially to ll the porcelain enclosure with a dielectric liquid which impregnates and lls thevoids in the solid insulation between the 'dilerent potential electrodes or other partsv in the' device. Whena conventional; porcelain-clad liquid lled transformer fails internally, the resultant electric arc decomposes the liquid and solid` insulation and .there is arapi'd increase in the amount of gas which is a product of such decomposition. This causes the production of veryA high pressures, and,` in fact, pressure waves have been ,observed having a peak value in excess of one-thousandA pounds per square inch-and thefmaximum value of these pressure vWavesY is '*often reached in 1/iooo of a second-or less; This very steep front of the pressure Wavealmostx invariably cracks the porcelairr andwhat is worse-the energy in the expanding'gas accelerates the porcelain fragments to a highyvelocity with resulting extreme danger to nearby personnel and severe damage to adjacent equipment.. y

In accordance `with theinvention, there is provided an electric induction device of this type which, while perhaps notstrictly explosion-proof,-

may aptly be described as explosion-safe in that the pressure wave which reaches the porcelain is'y 2 Claims. (Cl. 175--358) greatly attenuated, thus very much reducing the i likelihood of the `porcelain being cracked 4and.

` n thereto'.

indicated by any suitable dielectric liquid such an explosion-safe porcelain-clad liquid filled instrument transformer.

The invention will be better understood from the following description taken in connection 'with the accompanying drawing and its scope will be pointed out in the appended claims.'v

In the drawing Fig. 1 is a vertical sectional View through a high voltage current transformer embodying the invention, Fig. 2 is a similar view of a modification, Fig. 3 is an oscillogram of a lhas been attenuated by the special liner.

Referring nowAv to the drawing and more particularly to Fig. 1, there is shown therein a high.v voltage current transformer comprising a cylin- 'i drical porcelain insulator I inside of which is mounted a core and coil assembly indicated generally at 2; The lower end of the porcelain cylinder I is provided with a metal mounting flange 3 which is attached thereto by any suitable bonding means'll. For closing the lower end of the cylinder I Va metal plate 5 is attached to the mounting ange or ring 3 by bolts 6, andthe entire device is mounted on any suitable support such as the hollowvcylindrical member 'I.

'I'he core and coil assembly 2 comprises a solid insulated high voltage winding 8 of relatively few turns which links a solid insulated low voltage Winding 9 which is wound on a circular magnetic core (not shown) L The leads for the high voltage winding 8 are contained in a sleeve I0 extendingupwardly from the winding 8 and they emerge from the upper end of the sleeve I0 as conductors I2 which are attached to the inner ends of bushings I 3Y which are mounted in the Walls of a closure member I4 for the top of the y device.

to a metal ring I5 which in turn is attached `by.

The bottom of the closure I 4 is sealed bolts I6 to atop mounting flange I1 which is attached to the porcelain bushing I in the same manner that the bottom iiange 3 is attached The entire device is iilled to the level asmineral oil or askerel. y

For protecting theyporcelain I from the effects of steep front high pressure waves in the liquid produced by an internal fault in the transformer, a special cylindrical` liner I8 is interposed between the porcelain I and the core and coil assembly 2. lThis cylindrical liner consists of an inner resilient relatively mechanically weak cylinder I8 .insider'a rigid mechanically strong outer cylinder 20. One such liner which has given good results has a wall one inch thick consisting of an inner cylinder three-fourths of an inch thick of soft relatively loosely wound paper which has not been impregnated with any hardened binder and an outer cylinder one-fourth of an inch thick of glass cloth which has been impregnated with a suitablel hardened synthetic resin or a polymerized solventless varnish. Y

The walls o-f this liner .or duplex cylinder act like a low-pass filter and decrease the steepness of the shock wave which reaches the porcelain wall. This action is shown by a comparison of Figs. 3 and 4, Fig. 3 being a pressurewaveinsde the liner I8 having a maximumvalue of..3,720 pounds per square inch. Fig. #l "shows what is left of the pressure wave of Fig. 3 after it gets through the liner I8. That is to say, Figfl illus- ,time the relatively strong and rigid outer cylin- .derl prevents or greatly reduces the transmistrates the pressure wave whiohactually Vreaches the porcelain I as a result of the initial Vpressure wave of Fig. 3 inside thetransformer. The timing dashes at the bottom .ofFigs Sande. represent intervals of 1/iooo of a second.r

`The spacebetween the cylinder yI and the liner I8 Ais effectively sealed, at both ends sothat none of the gas developed by anarcing fault can get to theinside wall of the porcelain. Thisis raccomplished by having the bottom of the liner I8 rest oln'the plate 5. This plate is provided-withy an annular fange 2| yto `which the liner I 8 is attached by bolts 22. The upper. seal is 'obtained though there is differential thermal expansion andl contraction of the porcelain I-and thelinerV I8. The seals are not liquid tight and the space between the porcelain I and the liner I8 is normally filled with liquid. The functionofthe seals isto prevent high pressure kgasin'any quantity from entering the annular space between the.`

porcelain I and the liner I8 atleast during the initial stageof a fault in the transformer.

y VFon-relievingthepressurewhich is built up in the transformer as a result of an internal fault, pressurerelief devices are ymounted at both the top 'and the bottom'of the transformer.v Thus the top ef the closure I4 comprises a frangible diaphragm which is clamped lbetween Vtwo rings 26 by screws 2'I. VTransverse bars 428 and 29 above and below the diaphragm serve 'toretain rings 3E 'having sharp edges lwhich bear-k against the diaphragm 25. These serve to cut the diaphragm in a number of places when thereis '55 At `the a substantial pressure difference between Vthe inside and the outside ofthetransformer. i i bottom of the transformer there is a'central hole in the plate 5 which is closedbya frangible diaphragm 3l which is attached to the plate 5 by a ring 32 and screws v33. This diaphragm isl ,30. by.` means ofa` gasketZS which isolampedinplace l byzr-neans of a ring 2.4 vattached by lbolts-to thev` larger ring I5. A further advantage-of this con-A. struction is Athat the seal Vis maintained evennotched or weakened at 3! so that it will readilyy break when there is a substantial pressure difl ference between the inside and the` outside of the transformer. 1

The primary winding structure 8 and the combined secondary winding and core structure 9 are mounted in a suitable frame 35 which securely n holds those parts in their proper relative position. -This frame is securely attached to the member 5 in any suitable manner such las by bolts.

The secondary winding has leads 35 which terminate in secondary winding terminals 31.

The operation of Fig. 1 in minimizing theefv,

sion of pressure to the poreclain I. Consequently even though the porcelain should be cracked by the initial shockwave, practically no sustained high pressure lwill initially be applied to the porcelain so that none of the broken particles of 'porcelain can be accelerated outward for any appreciable distance or with any real force. Furthermore, the action of the liner I8 in delaying the :application of pressure to the porcelain I givesftirne for 'the pressure relief -diaphragms 25 an-dvr-'zlIto rupture and, lof course, as soon as these diaphragms do rupture the pressure inside the liner I8 very rapidly falls to substantially latmospheric pressure. The'reason for having both ari-.upper anda ylower diaphragm is that electrical failures can occur anywhere inside ,thetransformer andin'order to relieve this'pressureas quickly as'possible it is desirable thatthe pressure relief :.diaphragmbe as close as possible tov thetlocation of the fault. Inactual practice vthe diaphragmv which is :nearestrthe fault will rupture first but the other Vone will almost invariably rupturegalso so kthat `the transformer will be open at: both ends and the pressure will be discharged very, rapidly from both ends.

While best yresults have been obtained withthe duplex. liner consisting `of an inner resilient cylinder .andan outer strong rigid cylinder, itwill be apparent that either one. of these cylinders alone or in combination with the pressure relief diaphragms would also make the ytransformer muchsa'ferthan a conventional porcelain clad liquid filled transformer, and, therefore, the invention inits broader aspect is not limited to the'two difl ferent character cylinders. A Y

In Fig. v'2 the transformer is of the so-called eye-bolt type because of the resemblance of its tprimary'winding structure 38 to an eye-bolt.'-

This structure has an elongated neck 39 which is inside a porcelain cylinder 40. Between the neck 39 andthe porcelain cylinder 4D isa protective liner lll 'which is similar to the liner I8 of Fig. 1

in that it comprises an Vouter strong and rigid cylinder '42 andan inner resilient cylinder 43. The liner is sealed at its ends to the ends of the porcelain lll] in an:r suitable mannen The combined secondary winding and core of I' the transformeris indicated at M. The secondary leads areibrought out to terminals'l in thel lowerpart of a housing 41 which is provided with cppositely .disposed pressure relief ...di-

aphrag'ms 4 8. The v'primary Vand secondary wind- V4ings of the transformer, as in Fig. 1, are, of

course, provided with suitable solid insulation` which is adequate to withstand maximum voltage stresses between the parts, but inv orderto further increase .the dielectric strength of the device it 'is substantially entirely filled with liquid up to a level 49 in a top enclosure V50-which is attached to the top of the porcelain bushing 50 in any suitable manner. closes the top vof the lmember' 50.

A top pressure relief device 5I,v

The primary winding has leads 52 which terminate in high voltage terminals '53.

In practice there is practically no potential diiierence between the high voltage terminals because the primary winding ordinarily has only a very few turns so that the top metal enclosure I4 of Fig. l and 5l! of Fig. 2 is ordinarily at line potential, whereas the metal members or closures at the opposite end of the porcelain bushings are normally at ground potential.

The operation of Fig. 2 is the same as the operation of Fig. l.

While there have been shownand described particular embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An explosion safe porcelain clad liquid filled high voltage transformer, comprising in combination, a normally vertical hollow cylindrical porcelain insulator, a metal closure for the bottom of said cylinder which is normally at substantially ground potential, low voltage terminals mounted on said closure, a metal closure for the top of said cylinder which is normally at line potential, high voltage line terminals mounted on said top closure, a transformer core and coil assembly having high and low voltage windings, said core and coil assembly being between said tcp and bottom closures and at least partially surrounded by said insulating cylinder, the high voltage winding being connected to the high voltage terminals and the low voltage winding being connected to the low voltage terminals, a protective liner for the porcelain cylinder comprising a duplex cylinder having a relatively thick inner zone of relatively soft insulating material and a relatively thin outer zone of hard-insulating material, said liner being sealed to the top and bottom of said porcelain cylinder, pressure relief diaphragms in said top and bottom closures, and a liquid insulator iilling all free space in said device up to a level in said top closure.

2. An explosion safe porcelain clad liquid iilled high voltage transformer, comprising in combination, a normally vertical hollow cylindrical porcelain insulator, a metal closure for the bottom of said cylinder which is normally at substantially ground potential, low Voltage terminals mounted on said closure, a metal closure for the top of said cylinder which is normally at line potential, high voltage line terminals mounted on said top closure, a transformer core and coil assembly having high and low voltage windings, said core and coil assembly being between the opposite ends of said top and bottom closures and at least partially surrounded by said insulating cylinder, the high voltage winding being con-V nected to the high voltage terminals and the low voltage winding being connected to the low voltage terminals, a protective liner for the porcelain cylinder comprising an inner relatively soft paper loosely packed multi-layer cylinder with walls about one inch thick inside a hard resin impregnated glass cloth multi-layer outer cylinder with walls about one-quarter of an inch thick, said liner being sealed to the top and bottom of said porcelain cylinder, pressure relief diaphragms in said top and bottom closures, and a liquid insulator iilling all'free 'space in said device up to a level in said top closure.

GUGLIELMO CAMILLI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,670,697 Treanor May 22, 1928 2,303,435 Chiles, Jr. et al Dec. 1, 1942 FOREIGN PATENTS Number Country Date 441,161 Great Britain Jan. 14, 1936 568,055 Great Britain Mar. 15, 1945 

